1. Field of the Invention
The present invention relates to a polarizable electrode and an electric double layer capacitor using the same. More particularly, the present invention relates to an electric double layer capacitor suited to instantaneously supply an electric power, and a polarizable electrode used in the electric double layer capacitor.
2. Description of the Related Art
An electric double layer capacitor is a capacitor having a large capacitance to accumulate charges in an electric double layer. A toxicant material such as a heavy metal and the like is not used as the material of the capacitor. Therefore, a load to environment is small. Also, unlike a secondary battery, a chemical reaction is not brought about. Therefore, the electric double layer capacitor is superior in a cyclic life of a charging and discharging operation. For this reason, the electric double layer capacitor is widely used for a backup power supply of a micro-computer, a memory or the like, instead of the secondary battery.
In recent years, an activated carbon/polyacen composite material as disclosed in Japanese Patent No. 2054380 is invented. Also, an activated carbon layer has become possible to be formed on an aluminum foil by using a binder, as disclosed in Japanese Laid Open Patent Application (JP-A-Showa 57-60828). As a result, an electric double layer capacitor having a large capacitance and a low resistance can be manufactured.
In this way, the electric double layer capacitor starts to be applied to various fields needing power. As such application fields, there are energy regeneration in a hybrid vehicle using an engine along with a motor and an electric vehicle, relaxation of electric power generation variation of solar power generation, wind power generation, a backup power supply for instant power fault, and rush current supply when a motor is started.
In these application fields, it is needed that the electric double layer capacitor has a low resistance and a high voltage so that the power from hundreds of watts to tens of kilowatts can be charged and discharged in several seconds in a high voltage equal to or greater than 100 V.
FIG. 1 is a sectional view illustrating a conventional electric double layer capacitor. Referring to FIG. 1, the electric double layer capacitor is composed of a basic cell 6, terminal plates 7 which are provided on both sides of the basic cell 6 and in which solder is plated on an aluminum foil, sets of bolt and nut, and insulation bushes 8. The basic cell 6 is composed of collector electrodes 3, a separator 4, gaskets 5 and polarizable electrodes 9. The polarizable electrodes 9 function as positive and negative electrodes. The separator 4 prevents formation of an electric short circuit between the polarizable electrodes 9 and allows ions to pass through the separator 4. The collector electrodes 3 are electrically connected to an external circuit and do not allow the electrolyte to pass through the collector electrodes 3. The gaskets 5 are used to insulate the collector electrodes 3 from each other and seals the electrolyte. There may be an electric double layer capacitor having a coin cell structure or a winding structure as electric double layer capacitor using organic electrolyte.
The operation voltage of the electric double layer capacitor is limited to a voltage range in which any electrochemical reaction does not occurs in the polarizable electrode and the collector electrode and further the electrolyte is not resolved. Therefore, the operation voltage is about 1 V in a solution type of electrolyte, and about 2.5 V in an organic type of electrolyte. Usually, it is necessary to connect the basic cells 6 in series based on a desired operation voltage so as to increase a breakdown voltage. Therefore, tens to hundreds electric double layer capacitors must be connected in series in the power application field in which the breakdown voltage equal to or higher than 100 V is required.
Typically, the polarizable electrode 9 of the electric double layer capacitor is composed of activated carbon. The same material is used for both of the polarizable electrodes 9. The polarizable electrode 9 may be formed as a paste type electrode in which activated carbon powder or activated carbon fiber and electrolyte are mixed. Instead, the polarizable electrode 9 may be formed as a cloth type electrode of activated carbon fibers or resin fiber holding the activated carbon powder. Otherwise, the polarizable electrode 9 may be formed as a solid type electrode in which activated carbon powder and carbon organic are coupled by binder such as Teflon, as disclosed in Japanese Patent Publication (JP-B-Heisei 7-70448). Also, an activated carbon/polyacen composite material are well known as the material of the binder portion for realization of capacitance. As disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 8-97102), the polarizable electrode is also well known in which a part of the polarizable electrode is coated with carbon glass in order to prevent particles from falling off.
As the material of the collector electrode 3, conductive rubber, conductive elastomer, conductive polymer or the like is used as the solution type electrolyte, and metal such as aluminum or the like is used as the organic type electrolyte.
A porous film made of plastic and non-woven cloth are used as the material of the separator 4.
Conventionally, the electric connection between the polarizable electrode 9 and the collector electrode 3 is held by pressing with an outer case or bonding with organic binder. For example, a polarizable electrode and a collector electrode are bonded by using conductive adhesive, in Japanese Laid Open Patent Applications (JP-A-Heisei 7-161589, JP-A-Heisei 9-270370, and JP-A-Heisei 8-97102). A polarizable electrode and a collector electrode are formed as a unit by use of organic binder in Japanese Laid Open Patent Application (JP-A-Heisei 8-148388).
An example is disclosed in Japanese Laid Open Patent Application (JPU-A-Heisei 3-73426), in which carbon whiskers are used to couple a polarizable electrode and a collector electrode. Also, an electric double layer capacitor in which an collector electrode is in contact with a sintered polarizable electrode through a conductive metal film is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 3-87010). Further, a technique of pressing an collector electrode and the powder of activated carbon deposited on the collector electrode together with binder by a pressing machine is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 6-196364). In addition, an electrode structure in which fibers of activated carbon are disposed between an collector electrode and an electrode material by electrostatic planting is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 7-320987).
The resistance of the electric double layer capacitor is composed of the resistance of the polarizable electrode, the resistance of the electrolyte, the contact resistances between the component materials and the resistance of the electrolyte within the polarizable electrode. In order to reduce the resistances of the respective component materials, it is necessary that the resistivity of each component material is reduced, an area thereof is made wider and the thickness thereof is made thinner. The contact resistance can be reduced by increasing a contact area between the component materials. The resistance of the electrolyte within the polarizable electrode is different from the intrinsic resistance of the electrolyte. This is because movement of ions can not follow the change of an applied voltage in a macro pore having a small pore diameter in a sub-micron order.
Such a macro pore contains not only small pores of the activated carbon particle, but also gaps between the activated carbon particles and between the fibers. This resistance can be considered as a diffusion resistance, since the movement of the ions to the electric double layer capacitor is constricted by the shape of the small pore, similarly to a case that the movement of substance functions as a rate controlling parameter in an electrode reaction.
In order to reduce the diffusion resistance of the ions, it is necessary that the ions can move in a wider sectional area and movement distance of the ions are made shorter within the polarizable electrode. That is, it is enough to make the small pore diameter of the macro pore larger and to make the polarizable electrode thinner. However, if the number of macro pores is increased, a capacitance per volume is reduced. Thus, an optimization is required on the basis of a value of a charging and discharging current to be used.
As described above, the polarizable electrode is important to satisfy the following conditions. That is, the volume of the macro pore is as large as possible to the extent that an capacitance per volume is not decreased, the apparent electrode area is wide, the thickness of the electrode is thin and the polarizable electrode is in densely contact with the collector electrode. In addition, it is desirable that the electric double layer capacitor has the laminate structure such that the electric double layer capacitors can be connected with a low resistance in the series connection of them for a high voltage application.
In order to reduce the contact resistance between the polarizable electrode and the collector electrode made of the activated carbon, the method is conventionally adopted in which a pressure is applied from an external portion, or organic binder or conductive adhesive is used. The pressure applied from the external portion must be made higher in the case of the polarizable electrode of a paste type, a cloth shape, and a solid shape using organic binder. In this case, usually, the applied pressure becomes tens of kg/cm.sup.2. Thus, the outer portion becomes large to keep the pressure in case of the wide electrode area. This results in a defect that an energy density per capacitor volume or weight is reduced.
The solid type polarizable electrode coupled with carbon or made of the activated carbon/polyacen composite material is higher in rigidity. Accordingly, a high flatness is required for the contact surface with the collector electrode. This leads a problem that a forming process and a polishing process are required in a high accuracy so that manufacturing cost becomes expensive.
Further, in the conventional polarization electrode, the macro pore has a three-dimensionally complex structure. Even if the volume of the macro pore is mad e larger, the movement distance of an ion is long within the polarizable electrode. Therefore, it is impossible to achieve an excellent effect that the diffusion resistance can be reduced.
Furthermore, in the polarizable electrode using organic binder, there are problems that the macro pore is clogged by the organic binder particles and that the surface area of the activated carbon particle is made narrower.